In a principal aspect, the present invention relates to an improved resistor subassembly for use in an immersion heating element in an electric water heater tank, the tank having a protectively coated interior and a sacrificial anode immersed therein.
Electric water heaters or tanks have typically been provided with a protective magnesium rod which is adapted to be immersed in the water in the interior of the tank. This magnesium rod, or sacrificial anode, is installed to protect the interior wall of the tank against the formation of rust in the even of an imperfection in the protective coating of the tank wall. The sacrifical anode protects the lining of the tank by setting up galvanic currents between itself and the unprotected portion of the tank wall, thereby causing magnesium from the rod to become deposited upon the unprotected portion of the tank. This plating action thus protects the interior of the tank against corrosion.
A problem associated with the use of magnesium sacrificial anodes in water heater tanks having conventional electric immersion heating elements is that the magnesium rod is rapidly consumed due to the galvanic action set up between the rod and the heating element. This galvanic action effects transference of magnesium from the rod to the heating element, since the bare metallic sheath of the heating element presents a considerable surface area. Thus, the magnesium rod is rapidly dissipated.
To protect the magnesium rod from rapid dissipation and the resulting loss of the tank wall protection relatively early in the tank's useful life the heating element is eletrically insulated from the tank wall. This insulation breaks the galvanic circuit between the magnesium rod and the heating element electrically connected to the tank and thereby preserves the life of the magnesium rod.
A more serious problem results, however, from the insulation of the heating element from the tank wall. A certain amount of galvanic current flow still passes between the magnesium rod and the heating element, the current completing the circuit between the heating element and the tank wall by passing through the water in the tank. This results in serious corrosion of the heating element sheath and early failure of the heating element when the corrosion reaches the sheath.
Thus, it is advantageous to allow a small, predetermined amount of galvanic current flow between the heating element sheath and the tank wall of the water heater tank. This is accomplished typically by operatively interposing a resistor between the heating element sheath and the tank wall.
In a typical resistor-type immersion heater element, complex schemes to interpose a resistor between the tank wall and the heating element sheath have been suggested. Examples of these prior art resistor immersion heater elements are disclosed in U.S. Pat. No. 2,723,340, issued to Boggs et al. on Nov. 8, 1955, and U.S. Pat. No. 3,414,707, issued to Aldous on Dec. 3, 1968. These prior art resistor immersion heater elements employ multiplicity of specialized parts and are exceedingly difficult to construct and maintain.
These prior art devices, because of their complex design, are relatively fragile and easily broken. When repair becomes necessary, it may be less expensive to replace the entire heating element rather than the individual subassembly that becomes inoperative.
The present invention constitutes an improved resistor subassembly for a resistor-type immersion heating element that seeks to overcome the problems discussed while at the same time providing a simple, easily constructed design that is readily adaptable to a variety of immersion heating elements.